In this work, the interaction of components in the CO(NH2)2 - HOOC - COOH • NH2C2H4OH - H2O system was studied by visual polythermic method in the temperature range from -11.2 to 61°C. A polythermal solubility diagram of the system was constructed and the crystallization sites of ice, urea, monoethanolammonium oxalate, and the new phase ureamonoethanolammonium oxalate were determined. During the addition of monoethanolammonium oxalate to urea solutions of different concentrations, changes in rheological properties (pH, density, refractive index, viscosity) were recorded, and the results were found to be consistent with the polythermal solubility diagram. A new phase formed during the study of the CO(NH2)2 - HOOC - COOH • NH2C2H4OH - H2O system, ureamonoethanolammonium oxalate, was isolated and analyzed by confirmed using appropriate analytical methods (IR, Thermal, X-ray phase). [ABSTRACT FROM AUTHOR]